The present invention relates to a control system for a variable speed hydro-power plant apparatus, and more particularly to a control system for a variable speed hydro-power plant apparatus which is well suited to a case where a pump turbine or a water turbine being a driver exhibits a S-shaped characteristics in a turbine operation region.
The present invention relates to a control system for a variable speed hydro-power plant apparatus, and more particularly, it provides a control method well suited to a variable speed pumping-up power plant apparatus which shared an upstream or downstream side pipe line with another hydraulic machinery and which comprises a pump turbine exhibiting a S-shaped characteristics in a turbine or a water turbine operation region.
A variable speed hydro-power plant apparatus to which the present invention is applied uses a wound-rotor induction generator as an induction machine which is driven by a pump turbine or a water turbine (herein-below, simply termed "pump turbine"), and it excites the secondary winding thereof A.C.-wise through a frequency converter.
The variable speed hydro-power plant apparatus is therefore noticed owing to the feature that any desired rotating speed of the pump turbine can be established in the state in which the output frequency of the generator is held at an A.C. power line system frequency, so the variable speed hydro-power plant apparatus can be operated at a rotating speed affording a high pump turbine efficiency, or that when the generator is subjected to a variable speed pumping-up operation as a motor, an AFC (autmoatic frequency control) responsive operation is possible in spite of the operation as a simple constant speed motor.
Meanwhile, even in the case of the pump turbine of this variable speed hydro-power plant apparatus, a S-shaped characteristics turbine region is involved likewise to a conventional fixed speed pump turbine, and a measure for avoiding the S-shaped characteristics turbine region is indispensible.
Japanese Patent Application Laid-open No. 72998/1984 has proposed one technique for avoiding the entry of an operation into the unstable turbine operation region of the S-shaped characteristics at the stage of shift from no load to a rated load in the variable speed hydro-power plant apparatus.
In this prior art, in order to prevent the operating point of the pump turbine from rushing into the S-shaped characteristics turgine region, the guide vane opening degree control of the pump turbine is preferred to a rotating speed control based on the induction generator.
More specifically, the output of the induction generator is controlled according to a generator output command which is given to the generator, while the opening degree of the guide vane is first led to an opening degree corresponding to the changed generator output command as quickly as possible, and the appropriate rotating speed control is thereafter performed slowly.
The technique has the effect that the operation can be hindered from entering the unstable turbine operation region of the S-shaped characteristics at the start of the pump turbine.
In the conventional variable speed hydro-power plant apparatus according to Japanese Patent Laid-Open No. 71497/1987, the situations of fluctuations in the various quantities of the individual parts in the case where the generator output command P.sub.0 is increased are as shown in the section (a) in FIG. 9. It can be understood that, after the change of the generator output command P.sub.0 the various quantities become stable under new operating states. Referring now to FIG. 9, the fluctuations of the various quantities of the individual parts in transient states will be explained.
The generator output command P.sub.0 is now raised stepwise as shown in the section (a) in FIG. 9, at a point of time t.sub.0 with the intention of raising the generator output P as shown at the section (g) in FIG. 9. Then, the generator output P of the induction generator rises following up the change of the generator output command P.sub.0 as shown at the section (a) in FIG. 9.
On the other hand, after the generator output command P.sub.0 has been given, the response of the opening degree Y of the guide vane is slower than the response of the generator output P. For this reason, the turbine output P.sub.T shown at the section (e) in FIG. 9 becomes smaller than the generator output P, and the rotating speed N shown at the section (f) in FIG. 9 is temporarily decelerated after the sudden change of the generator output command P.sub.0.
Thereafter, at a point of time t.sub.1, the generator output P and the turbine output P.sub.T become equal, and the rotating speed N becomes the minimum. At this time t.sub.1, the rotating speed deviation or correction signal .DELTA.N=(Na-N) is plus, so that the guide vane opening degree deviation or correction signal .DELTA.Y=(Ya-Y) is plus, and the guide vane opening degree Y increases more than the appropriate guide vane opening degree command Y.sub.a shown at the section (b) in FIG. 9.
Accordingly, the turbine output P.sub.T becomes greater than the generator output P, and the rotating speed N begins to rise as shown at the section (f) in FIG. 9. With the rise of the rotating speed N, the rotating speed deviation .DELTA.N thereof from the appropriate rotating speed command N.sub.a decreases, and with the decrease of the guide vane opening degree deviation .DELTA.Y, the turbine output P.sub.T decreases, and the acceleration of the rotating speed N decreases.
The rotating speed deviation .DELTA.N in a steady state is rendered zero by the integral element in the rotating speed control unit. On the other hand, the guide vane opening degree deviation .DELTA.Y between the appropriate guide vane opening degree command Y.sub.a from the turbine characteristics function generation unit and the guide vane opening degree Y corresponds to the error between a characteristics of the pump turbine, and it can be rendered almost zero by enhancing the precision of the function of the turbine characteristics.
Consequently, the integral element in the rotating speed control unit may generate only the guide vane opening degree deviation .DELTA.Y=(Y.sub.a -Y) in the steady state.
The above will be explained again by the use of equations.
The secondary excitation unit has an integral element etc. built therein for the purpose of adjusting the generator output P to the generator output command P.sub.0. In the steady state, EQU P=P.sub.0 ( 1)
Owing to the integral element built in the rotating speed control unit, in the steady state, EQU N=N.sub.a ( 2)
Besides, owing to the guide vane drive unit, in the steady state, EQU Y=Y.sub.a +.DELTA.Y (3)
Moreover, in the steady state, the turbine output P.sub.T and the generator output P ought to be equal: EQU P=P.sub.T =f(H, Y, N) (4)
Further, the appropriate guide vane opening degree command Y.sub.a ought to have originally been given so as to correspond to the generator output command P.sub.0 under the water head or effective head H on that occasion and N=N.sub.a. Therefore, EQU P.sub.0 =f(H, Y, N) (4)
With the above put together, Y=Y.sub.a holds, namely, the guide vane opening degree deviation .DELTA.Y=0 holds in the steady state. In the rotating speed control unit, the gain (K.sub.1) of the proportional element having a braking effect is made greater, and the gain (K.sub.2) of the integral element is made relatively smaller, thereby to raise the response rate of the opening degree of the guide vane to the utmost. On the other hand, the turbine output P.sub.T and the rotating speed N are set without vibrations as seen from at the sections (a) and (f) in FIG. 9.
Thus far, the responsive operations of the prior art variable speed hydro-power plant apparatus have been explained. In brief, according to this prior art variable speed hydro-power plant apparatus, the turbine input and the generator output are controlled in balanced fashion, whereby the various quantities can be held stable.
Now, the unstable operation phenomenon of the pump turbine will be explained.
In general, the runner and other devices of the pump turbine, especially a high water head pump turbine, are so designed as to demonstrate a sufficient centrifugal pumping action to the end of attaining a high water head during a pumping operation.
The design, however, adversely affects the operation of the pump turbine. In particular, it is considered inevitable that the turbine characteristics called "S-shaped characteristics" is involved.
In a case where the characteristics of the pump turbine adopted for this design is indicated by a characteristics curve which expresses the relationship between a rotating speed per unit head (N.sub.1) and a flow rate per unit head (Q.sub.1) under the predetermined opening degree of the guide vane, the characteristics curve has in the operation region of the pump turbine, the first part in which the value of the flow rate per unit head (Q.sub.1) decreases with increase in the value of the rotating speed per unit head (N.sub.1) and the second part in which the value of the flow rate per unit head (Q.sub.1) decreases with decrease in the value of the rotating speed per unit head (N.sub.1).
In order to facilitate the description, the second part shall be termed "S-shaped characteristics part" in this specification. Further, the characteristics of the pump turbine in the S-shaped characteristics part shall hereinafter be termed the "S-shaped characteristics".
During the turbine operation in the S-shaped characteristics part, also a torque per unit head (T.sub.1) decreases with the decrease of the rotating speed per unit head (N.sub.1).
Usually, the operation of the pump turbine is performed in the first part. However, in a case where the rotating speed per unit head (N.sub.1) increases suddenly and greatly due to an abrupt load decrease or the like, there is the possibility that the pump turbine will rush into the S-shaped characteristics part or region. Once the pump turbine has rushed into the S-shaped characteristics part or region, it becomes, in effect, impossible to continue the operation of the variable speed hydro-power plant apparatus.
The characteristics of the pump turbine having the S-shaped characteristics in the turbine operation region are illustrated in FIG. 10 and FIG. 11. In FIG. 10, the characteristics of the pump turbine is shown as the relationship between the rotating speed per unit head (N.sub.1) and the flow rate per unit head (Q.sub.1) with the guide vane opening degree (Y) taken as a parameter.
On the other hand, in FIG. 11, the characteristics of the pump turbine is shown as the relationship between the rotating speed per unit head (N.sub.1) and the torque per unit head (T.sub.1) by the use of the same parameter. EQU N.sub.1 =N/.sqroot.H, Q.sub.1 =Q/.sqroot.H, T.sub.1 =T/H
In the above expressions, symbols N, Q, H and T denote the rotating speed, the flow rate, the effective head and the torque of the pump turbine, respectively.
Characteristics curves 1 and 1' are obtained under a comparatively great guide vane opening degree as predetermined. Characteristics curves 2 and 2' are obtained under a smaller guide vane opening degree. Characteristics curves 3 and 3' are obtained under a still smaller guide vane opening degree.
In the part a-d-h of the characteristics curve 1, the value of the flow rate per unit head (Q.sub.1) decreases with the decrease of the rotating speed per unit head (N.sub.1). As stated above, this curve part a-d-h is termed as the "S-shaped characteristics part" in this specification.
Similarly, a curve part b-e-i is the S-shaped characteristics part of the characteristics curve 2, and a curve part c-f-j is that of the characteristics curve 3. Such S-shaped characteristics turbine regions shift to the higher the rotating speed per unit head (N.sub.1) side as the guide vanes become larger.
Likewise to the curve parts in FIG. 10, also in FIG. 11, curve parts a'-d'-h', b'-e'-i' and c'-f'-j' are the S-shaped characteristics parts of the characteristics curves 1', 2' and 3', respectively.
FIG. 11 has close relations with FIG. 10. For example, a point x meeting Q.sub.1 =Q.sub.1x and N.sub.1 =N.sub.1x on the characteristics curve 3 in FIG. 10 corresponds to a point x' on the characteristics curve 3' in FIG. 11. The point x' is a point meeting T.sub.1 =T.sub.1x' and N.sub.1 =N.sub.1x' (=N.sub.1x). Similarly, points a, b, c, d, e, f, h, i, and j in FIG. 10 correspond to a', b', c', d', e', f', h', i' and j' in FIG. 11, respectively.
A curve nr is a no-load flow rate curve. The intersection points .alpha., .beta. and .gamma. between the characteristics curves 1, 2 and 3 and the no-load flow rate curve nr correspond to the intersection points .alpha.', .beta.' and .gamma.' between the characteristics curves 1', 2' and 3' and a straight line T.sub.1 =0, respectively.
The variable speed hydro-power plant apparatus is operated under conditions differing every moment, and it is sufficiently presumed that the operation approaches or enters the unstable turbine operation region during an ordinary operation except the start operation. Nevertheless, the above stated prior art is not effective as a measure of avoidance and extrication in this case.
Again, in the above stated Japanese Patent Laid-Open No. 72998/1984, while the output of the generator is controlled according to a generator output command given to the generator, the opening degree of guide vane is led to an opening degree corresponding to the changed generator output as quickly as possible, and the appropriate control of a rotating speed is thereafter performed slowly.
In actuality, however, the control rate of the guide vane opening degree is inevitably limited lest abnormal water hammering should arise in a waterway upstream or downstram of the pump turbine, and it can never be raised up to the response rate level of the generator output control.
On the other hand, suppressing the response rate of the generator output control is none other than sacrificing the degree of contribution of the variable speed hydro-power plant apparatus to an electric power system and is truly unfavorable. As a result, it is proper to consider that the rotating speed greatly fluctuates transiently. That is, in the above stated prior art, the rotating speed lowers when the load increases, and it rises when the load decreases.
Assuming now that the operating point (for example, the rotating speed per unit head N.sub.1 =N/.sqroot.H) of the pump turbine before the load change lies near the S-shaped characteristics turbine operation region, there is no problem at the load increase because the turbine operating point moves in the direction of coming away from the S-shaped characteristics turbine operation region while accompanying the lowering of the rotating speed.
To the contrary, at the load decrease, the turbine operating point comes nearer while accompanying the rise of the rotating speed, and it can rush into the S-shaped characteristics turbine operation region in some cases.
Once the turbine operating point has rushed into the S-shaped characteristics turbine operation region, the torque of the pump turbine lowers rapidly and greatly together with the flow rate of water, and the rotating speed lowers rapidly. At this turbine operating point of time, it becomes, in effect, impossible to continue the normal operation of the variable speed hydro-power plant apparatus.
Further, notice needs to be taken of the fact that the above state inrush of the turbine operating point into the S-shaped characteristics turbine operation region does not take place due to only the transient change of the rotating speed (N). The rotating speed per unit head N.sub.1 =N/.sqroot.H increases also when the effective head (H) has lowered for any reason.
Especially in a case where the upstream or downstream pipe line of the pump turbine is shared with another water turbine or pump turbine, the water hammering phenomenon which has arisen in the other water turbine or pump turbine is propagated through the shared pipe line. Accordingly, even when only the variable speed hydro-power plant apparatus is perfectly controlled, there is the potential that the problem stated above will be posed by another apparatus.
Japanese Patent Laid-Open No. 164080/1986 proposes a method in which, when a variable speed hydro-power plant apparatus is to be put on an electric power system, the rotating speed is set at the minimum value allowable under the effective head (H) on that occasion, whereby the induction generator is stably put on the electric power system without the influence of the S-shaped characteristics turbine operation region.
However, nothing is referred to as regards a method of avoiding inrush into the S-shaped characteristics turbine operation region, the method being effective after putting the unit on the electric power system. In particular, in the variable speed hydro-power plant apparatus, the rotating speed is quite independent of the frequency of the electric power system unlike those of a fixed speed apparatus (a synchronous generator).
That is, a synchronizing torque from the electric power system cannot be expected. In other words, even after putting on the electric power system, just as before it, the control of the rotating speed must be rendered satisfactory by the variable speed hydro-power plant apparatus itself. In this respect, Japanese Patent Laid-Open No. 164080/1986 is incomplete.
The present invention consists in eliminating the afore-mentioned disadvantages of the prior art techniques, and providing a simple and reliable method of controlling a variable speed hydro-power plant apparatus in which, even when the influence of water hammering is exerted from another hydraulic machinery sharing a pipe line, a turbine operating point N.sub.1 =N/.sqroot.H (where N denotes a rotating speed, and H an effective head) is prevented from abnormally approaching the S-shaped characteristics turbine operation region.